Traditionally, when receiving waveforms for processing by a test instrument, such as an oscilloscope, the waveforms are acquired as a number of discrete acquisitions, the timing of the start and/or end of each acquisition being defined in accordance with a trigger signal. The trigger signal is typically asynchronous with the sampling process and generated in accordance with an external signal, clock or frame synchronization signal. The difference in time of the moment of the trigger and the sampling process is measured, and thus, each acquired waveform's samples are assigned to a particular phase, relative to the trigger signal. These typically brief waveforms, or data acquisition segments, are either stored for later processing, or may be processed immediately. However, this acquisition scheme has a number of significant drawbacks when used in a high-speed data acquisition environment. The quality and consistency of the data acquisitions are dependent on the precision of the timing measurement relating the trigger signal to the timing of the sampling clock used to store the data signal. Any imprecision in the timing measurement between the two, known as trigger jitter, will result in the phase of the discrete data signal acquisitions to be out of synchronization with the true phase of the data signal, as well as with the phase of the other data acquisitions, thus limiting the ability to compare various ones of the data signal acquisitions, and moreover limiting the ability to correctly characterize the signal.
Additionally, the generation of a precisely accurate clock signal is made harder when a high-speed data acquisition with a substantially large number of bits is to be acquired. The high-speed nature of the acquisition requires a very fast clock signal, thus compounding the possibility for jitter. Because of the extremely short acquisition times, any clock jitter becomes a larger percentage of the acquisition time, thus resulting in an even larger phase difference between the various acquisition data segments. While it would be possible to use a slower clock signal and leave out some data from the data signal so that all of the data is not stored and processed, this method is undesirable because, for example, a non-recurring error may be encountered but not properly identified and processed because all of the data will not be considered.
An additional limitation with this method is that a dead-time is incurred between each signal acquisition, while the time measurement apparatus and storage apparatus are prepared to record another acquisition. In the context of measurements performed on a serial data signal, the dead-time inherent in this method prohibits the analysis of immediately adjacent data bits as distinct waveforms, or for that matter a long stream of contiguous data bits as distinct waveforms.
Therefore, an improved method and apparatus for acquiring and processing an acquired data signal for analysis in a test instrument, such as a digital oscilloscope, is desired.